The present invention relates to the manufacture of SZ-twisted strands or braids of such strands in general, and particularly to the manufacture of such strands or braids for use in telecommunication cables.
There is already known, for instance, from the German Pat. No. 2,138,239, a method of and an apparatus for SZ twisting of individual elongated elements each of which may be constituted by a bunch of non-twisted or twisted filaments, particularly for use in telecommunication cables. It is also already known from this patent to advance the elongated elements longitudinally thereof, after withdrawing the elements from stationary supplies, and to twist the elongated elements by resorting to the use of a twisting head which is arranged between an inlet nipple and an outlet nipple, to thereby obtain a twisted strand of the elements which is subsequently wound on stationary drums, reels or other storage equipment. In this prior-art arrangement, the twisting head is stationary but for its continuous rotation at the same twisting speed and in the same twisting direction, while a spacing element which keeps the individual elongated elements at a distance upstream of the inlet nipple is cyclically displaced in and contrary to the advancement direction of the elongated elements at a displacement speed which is lower than the advancement speed of the elongated elements, thereby cyclically varying the distance of the spacing element from the inlet nipple. On the other hand, the distance between the twisting head and the outlet nipple remains constant.
It is also already known from the above-mentioned patent to unite a plurality of the strands obtained in the above-mentioned manner into a braid. In this connection, it has been proposed in order to electrically decouple the twisted elements of the braid, to vary the displacement speed of the individual spacing elements with respect to one another. These variations in the displacement speeds can be accomplished solely as a function of the location, that is, by individually controlling the displacement speeds of the individual twisting arrangements, or as a function of both the location and the time. In the first instance, the displacement speed is constant for each of the strands which are to be united into the braid, but the displacement speed of each of the spacing elements is different by a certain factor from those of all of the other spacing elements. This results in a situation where the lengths of the same-twist zones of the same strand are the same, but such lengths are different for each of the strands. In the second instance, the displacement speeds of the individual separating elements associated with the respective strands will somewhat vary in time independently of one another.
Experience with the above-discussed arrangement has shown that twist-free regions develop at the transition from a twisted zone of one twisting direction to the twisting zone of the other direction, as a result of the fact that the direction of displacement of the spacing element is reversed at the two terminal positions of the trajectory of displacement of the spacing element. Of course, such twist-free regions are undesirable in the twisted strand or cable. On the other hand, it has also been established by experience that, when it is desired to stagger the transition regions at which the twist of the individual strands takes place in accordance with the prior-art proposals, it is impossible to displace the respective spacing elements from a single driving arrangement, because of the different displacement speeds of the individual spacing elements. Rather, it is necessary to arrange transmissions of a step-up or step-down type between the individual spacing elements, as a result of which undesirably high expenditures are incurred.